Device and method for wireless power transfer

ABSTRACT

A power transfer device is a power transmitter ( 201 ) or a power receiver ( 205 ) conducting power transfer using an electromagnetic power transfer signal employing a repeating time frame comprising a power transfer time interval and an object detection time interval. A power transfer circuit ( 303, 307 ) comprises a power transfer coil ( 203, 207 ) receiving or generating the power transfer signal during the power transfer time intervals. A communicator ( 315, 323 ) communicates with the other device via an electromagnetic communication signal. A communication resonance circuit ( 317, 321 ) comprises a communication antenna ( 319, 325 ) for transmitting or receiving the electromagnetic communication signal. During the communication, the communication resonance circuit ( 317, 321 ) provides a resonance at a first resonance frequency to the communicator ( 315, 323 ). A controller ( 333, 335 ) adapts the communication resonance circuit to not provide the resonance at the first resonance frequency to the communicator during object detection time intervals. The approach may provide improved detection of resonance objects, such as smart cards (e.g. NFC cards).

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is the U.S. National Phase application under 35 U.S.C.§ 371 of International Application No. PCT/EP2019/083212, filed on Dec.2, 2019, which claims the benefit of EP Patent Application No. EP18210392.9, filed on Dec. 5, 2018. These applications are herebyincorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to wireless power transfer and in particular, butnot exclusively, to high power level power transfer, such as for examplefor supporting kitchen appliances.

BACKGROUND OF THE INVENTION

Most present-day electrical products require a dedicated electricalcontact in order to be powered from an external power supply. However,this tends to be impractical and requires the user to physically insertconnectors or otherwise establish a physical electrical contact.Typically, power requirements also differ significantly, and currentlymost devices are provided with their own dedicated power supplyresulting in a typical user having a large number of different powersupplies with each power supply being dedicated to a specific device.Although, the use of internal batteries may avoid the need for a wiredconnection to a power supply during use, this only provides a partialsolution as the batteries will need recharging (or replacing). The useof batteries may also add substantially to the weight and potentiallycost and size of the devices.

In order to provide a significantly improved user experience, it hasbeen proposed to use a wireless power supply wherein power isinductively transferred from a transmitter coil in a power transmitterdevice to a receiver coil in the individual devices.

Power transmission via magnetic induction is a well-known concept,mostly applied in transformers having a tight coupling between a primarytransmitter inductor/coil and a secondary receiver coil. By separatingthe primary transmitter coil and the secondary receiver coil between twodevices, wireless power transfer between these becomes possible based onthe principle of a loosely coupled transformer.

Such an arrangement allows a wireless power transfer to the devicewithout requiring any wires or physical electrical connections to bemade. Indeed, it may simply allow a device to be placed adjacent to, oron top of, the transmitter coil in order to be recharged or poweredexternally. For example, power transmitter devices may be arranged witha horizontal surface on which a device can simply be placed in order tobe powered.

Furthermore, such wireless power transfer arrangements mayadvantageously be designed such that the power transmitter device can beused with a range of power receiver devices. In particular, a wirelesspower transfer approach, known as the Qi Specifications, has beendefined and is currently being developed further. This approach allowspower transmitter devices that meet the Qi Specifications to be usedwith power receiver devices that also meet the Qi Specifications withoutthese having to be from the same manufacturer or having to be dedicatedto each other. The Qi standard further includes some functionality forallowing the operation to be adapted to the specific power receiverdevice (e.g. dependent on the specific power drain).

The Qi Specification is developed by the Wireless Power Consortium andmore information can e.g. be found on their website:http://www.wirelesspowerconsortium.com/index.html, where in particularthe defined Specification documents can be found.

In order to support efficient wireless power transfer, wireless powertransfer systems, such as Qi based systems, utilize substantialcommunication between the power transmitter and the power receiver.Initially, Qi supported only communication from the power receiver tothe power transmitter using load modulation of the power transfersignal. However, developments of the standard have introducedbidirectional communication and many functions are supported bycommunication exchanges between the power receiver and the powertransmitter. In many systems, the communication from the powertransmitter to the power receiver is accomplished by modulating thepower transfer signal. However, it has also been proposed to usecommunication functionality which is independent of the power transfersignal and which does not use the power transfer signal as a carrierbeing modulated. For example, the communication between powertransmitter and power receiver may be achieved by a short rangecommunication system such as RFID/NFC communication approaches.

Using a separate communication approach may in many situations provideimproved performance and may e.g. provide faster communication with ahigher communication reliability and a reduced impact on the ongoingpower transfer.

In power transfer systems, such as Qi, the electromagnetic fieldgenerated to transfer the required levels of power to the power receiveris often very substantial. The presence of such a strong field may inmany situations have an impact on the surroundings. For example, apotential problem with wireless power transfer is that power mayunintentionally be transferred to e.g. metallic objects that happen tobe in the vicinity of the power transmitter.

In order to reduce the risk of such scenarios arising, it has beenproposed to introduce foreign object detection where the powertransmitter can detect the presence of a foreign object and reduce thetransmitted power and/or generate a user alert when a positive detectionoccurs. For example, the Qi system includes functionality for detectinga foreign object, and for reducing power if a foreign object isdetected. Specifically, Qi specification version 1.2.1, section 11describes various methods of detecting a foreign object.

One method to detect such foreign objects is disclosed inWO2015018868A1. Another example is provided in WO 2012127335 whichdiscloses an approach based on determining unknown power losses. In theapproach, both the power receiver and the power transmitter measuretheir power, and the receiver communicates its measured received powerto the power transmitter. When the power transmitter detects asignificant difference between the power sent by the transmitter and thepower received by the receiver, an unwanted foreign object maypotentially be present, and the power transfer may be reduced or abortedfor safety reasons. This power loss method requires synchronizedaccurate power measurements performed by the power transmitter and thepower receiver.

For example, in the Qi power transfer standard, the power receiverestimates its received power e.g. by measuring the rectified voltage andcurrent, multiplying them and adding an estimate of the internal powerlosses in the power receiver (e.g. losses of the rectifier, the receivercoil, metal parts being part of the receiver etc.). The power receiverreports the determined received power to the power transmitter with aminimum rate of e.g. every four seconds.

The power transmitter estimates its transmitted power, e.g. by measuringthe DC input voltage and current of the inverter, multiplying them andcorrecting the result by subtracting an estimation of the internal powerlosses in the transmitter, such as e.g. the estimated power loss in theinverter, the primary coil, and metal parts that are part of the powertransmitter.

The power transmitter can estimate the power loss by subtracting thereported received power from the transmitted power. If the differenceexceeds a threshold, the transmitter will assume that too much power isdissipated in a foreign object, and it can then proceed to terminate thepower transfer.

Alternatively, it has been proposed to measure the quality or Q-factorof the resonant circuit formed by the primary and secondary coilstogether with the corresponding capacitances and resistances. Areduction in the measured Q-factor may be indicative of a foreign objectbeing present.

In practice, it tends to be difficult to achieve sufficient detectionaccuracy using the methods described in the Qi specification.

A particular issue has been identified as being the detection of SmartCards and similar items, such as e.g. NFC cards.

A contactless smart card is typically a small device for contactlesscommunication using electromagnetic coupling between a tuned antenna ofa reader and a resonance circuit of a receiver. In many situations, thesmart card is a passive device powered by the signal induced in theresonance circuit. This allows cheap cards to be produced and used witha range of readers. Often a smart card is used as a contactlesscredential and tend to be credit-card sized.

Typically, an embedded integrated circuit (chip) can store (andsometimes process) data and communicate with a reader via a suitableprotocol/standard, such as specifically using Near Field Communication(NFC). Commonplace uses include transit tickets, bank cards andpassports.

As illustrated by the example of FIG. 1 , an NFC smart card 101 mayelectrically typically comprise an antenna coil L, a tuning capacitor C,a rectifier 103 and an NFC chip 105 powered by the signal extracted bythe resonance circuit formed by the capacitor C and the coil L. In mostcases the parallel resonance of a smart card is tuned to a resonancefrequency of 13.56 MHz.

FIG. 1 also illustrates a simplified model of an NFC reader 107 forreading the data stored in the smart card. An NFC reader typicallycomprises an NFC reader chip 109 and an NFC antenna 111 which is alsotuned at a resonance frequency of 13.56 MHz. If the smart card 101 isbrought into the proximity of the NFC reader 107, the antenna coil L isexposed to the 13.56 MHz magnetic field from the NFC reader 107 and theNFC chip 105 powers up via the Vcc pin. Once powered up, the NFC chip105 in the smart card is able to modulate its own Vcc by means of loadmodulation thereby sending data back to the NFC reader 107. The timebetween powering up of the NFC chip in the smart card and the sending ofinformation back to the NFC Reader is typically in the range of about30-50 mSec.

Detection of such cards has been found to be particularly important dueto them being very popular and almost ubiquitous in many typical usageenvironments for wireless power transfer (e.g. in homes) and due to thesensitivity of such cards to the presence of strong electromagneticfields. Indeed, it has been found that the electromagnetic fieldstrength employed in many wireless power transfer systems, such asspecifically for higher power level transfers, in many scenarios maypotentially damage the cards. It has been found that smart cards may bedamaged if exposed to a strong AC magnetic field, even if the frequencyof that field is very different from the resonance frequency of 13.56MHz. If for instance a strong AC magnetic field in the order of 20-200kHz is exposed to the antenna coil L of FIG. 1 , the rectified voltageVcc might become too high and destroy the NFC chip 101. Furthermore, thetime required to damage the card may be much shorter than 30-50 mSec (ofcourse depending of the level of the AC magnetic field to which thesmart card is exposed). This means that the NFC chip inside the smartcard can be destroyed by a strong AC magnetic field.

It has at the same time been found to be particularly difficult todetect such cards using conventional foreign object detection approachessince they typically contain relatively small amounts of metal. Thismakes it very challenging to get a sufficiently high accuracy andreliability of the detection.

In order to address this problem, it has been proposed to introducedetectors specifically aimed at detecting the presence of smart cardsand similar devices. These smart card detectors are typically based ondetecting the presence of a resonance circuit having resonancefrequencies matching the expected resonance for a smart card.

Such dedicated detection has been found to provide improved detectionperformance allowing smart card detection in many scenarios. However, ithas been found that the reliability and accuracy of detection of smartcards in practice tend to be lower than preferred resulting inpotentially missed detections or false positive detections.

Hence, an improved power transfer approach would be advantageous, inparticular, an approach allowing increased flexibility, reduced cost,reduced complexity, improved communication, improved power transferoperation, improved object detection performance, a more reliable and/orsecure power transfer operation, reduced risk to other objects such asspecifically to smart cards and similar, improved accuracy and/orreliability and/or accuracy in detecting foreign objects such as smartcards or similar, and/or improved performance would be advantageous.

SUMMARY OF THE INVENTION

Accordingly, the Invention seeks to preferably mitigate, alleviate oreliminate one or more of the above mentioned disadvantages singly or inany combination.

According to an aspect of the invention there is provided a powertransfer device for wireless power transfer from a power transmitter toa power receiver using an electromagnetic power transfer signal, thepower transfer device being one of the power transmitter and the powerreceiver, and the power transfer signal during a power transfer phaseemploying a repeating time frame comprising a power transfer timeinterval and an object detection time interval being non-overlappingwith the power transfer time interval, a power limit of the powertransfer signal being lower for the object detection time interval thanfor the power transfer time interval, the power transfer devicecomprising: a power transfer circuit comprising a power transfer coilfor receiving or generating the power transfer signal during the powertransfer time intervals; a communicator for communicating with acomplementary device being the other device of the power receiver andthe power transmitter via an electromagnetic communication signal; acommunication resonance circuit comprising a communication antenna fortransmitting or receiving the electromagnetic communication signal, thecommunication resonance circuit during communication being arranged toprovide a resonance at a first resonance frequency to the communicator;and a controller for adapting the communication resonance circuit to notprovide the resonance at the first resonance frequency to thecommunicator during object detection time intervals.

The invention may allow improved operation in many embodiments and mayespecially allow improved, faster, and/or facilitated detection of, inparticular, objects exhibiting resonances to electromagnetic fields atfrequencies close to the first resonance frequency, where the detectionmay be performed by the power transfer device or the complementary powertransfer device. The power transmitter may perform object detectionbased on an assumption of the object comprising a resonance around thefirst resonance frequency (e.g. within 5 or 10%). The approach may allowfor this detection to be improved, e.g. allowing the detection to beperformed in short time intervals and/or with higher accuracy andreliability.

The approach may in particular provide an operation that reduces theinterference that a communication circuit may provide to a resonancebased object detection function which is based on an assumption of aresonance corresponding to (close to or the same as) the communicationresonance frequency/the first resonance frequency.

The approach may in particular allow improved detection of foreign smartcards, such as NFC smart cards, in a wireless power transfer systemwhich utilizes corresponding communication, e.g. NFC communicationbetween the power receiver and the power transmitter.

In many embodiments, a duration of the object detection time interval isno more than 5%, 10%, or 20% of the duration of the repeating timeframe. In many embodiments, the duration of the power transfer intervalis no less than 70%, 80%, or 90% of the time frame.

In many embodiments, the repeating time frame may further include atleast one communication time interval. The communication time intervalmay be non-overlapping with the power transfer time interval and theobject detection time interval. During a communication time interval,the power level of the power transfer signal may be subject to a lowermaximum limit than during the power transfer time interval. In manyembodiments, the power transfer signal may have a low power level duringcommunication time intervals and object detection time intervals, and ahigh power level during power transfer time intervals.

In many embodiments, the repeating time frame may be a periodicrepeating time frame. In many embodiments, each repeating time frame mayhave a duration of no more than 0.5 second, 1 second, 2 seconds, or 5seconds.

The timing of the object detection time intervals and/or the powertransfer time intervals may be independent of the power transferproperties, data exchange between the power transmitter and the powerreceiver, object detections, power requests, and power transferrequirements. The timing of the object detection time intervals may bepredetermined within a repeating time frame and/or may be the same inconsecutive repeating time frames. The relative timing of an objectdetection time interval within a repeating time frame may be unchangedbetween consecutive repeating time frames, and may be fixed. The timingof object detection time intervals within the repeating time frames maybe constant (for at least some consecutive time frames). In manyembodiments, a time frame may be divided into a plurality of timeintervals including at least one power transfer time interval and oneobject detection time interval. The timing of the time intervals withinthe repeating time frame (e.g. relative to a start or end time of therepeating time frame) may be the same for consecutive time frames.

The power receiver may be present and request power throughout the powertransfer phase. The power receiver may be detected/determined to bepresent (by the power transmitter) throughout the power transfer phase.

In many embodiments, the controller may control the communicationresonance circuit to have a resonance at the first resonance frequencyduring communication, and specifically during communication timeintervals, and to not have a resonance at the first resonance frequencyduring the object detection time intervals.

In accordance with an optional feature of the invention, the controlleris arranged to decouple the communication resonance circuit from thecommunicator during object detection time intervals.

In accordance with an optional feature of the invention, the controlleris arranged to detune the communication resonance circuit from the firstresonance frequency during object detection time intervals.

In accordance with an optional feature of the invention, the controlleris arranged to detune the communication resonance circuit to a secondresonance frequency during the object detection time intervals.

In accordance with an optional feature of the invention, the secondfrequency is outside a frequency range from 90% of the first resonancefrequency to 110% of the first resonance frequency.

In accordance with an optional feature of the invention, the controlleris arranged to change a resonance capacitance of the communicationresonance circuit during the object detection time interval relative toduring communication.

The above additional features may provide improved performance and/orfacilitated implementation in many embodiments. The detuning of thecommunication resonance circuit may correspond to modifying theresonance circuit to resonate at a different frequency or may correspondto modifying the resonance circuit to no longer have a resonance(corresponding to deactivating the resonance circuit).

In accordance with an optional feature of the invention, the firstresonance frequency deviates by no more than 5% from a carrier frequencyof the electromagnetic communication signal.

This may provide improved performance in many embodiments. In typicalembodiments, the first resonance frequency may be substantiallyidentical to the frequency of the electromagnetic communication signalthereby allowing optimized communication. However, in practice the firstresonance frequency may differ slightly from the communication frequencye.g. due to imperfect tuning resulting from component variances andtolerances.

In accordance with an optional feature of the invention, the powertransfer device is the power transmitter.

The invention may provide an improved power transmitter.

In accordance with an optional feature of the invention, the powertransfer device further comprises an object detector for detecting apresence of an object comprising a resonance circuit having a resonancefrequency corresponding to the first resonance frequency.

The invention may allow improved detection of devices/objects comprisingresonance circuits, such as for example NFC cards, by a powertransmitter. The object may comprise a resonance circuit having aresonance frequency assumed to be within 5% of the first resonancefrequency.

In many embodiments, the power transfer device may comprise an objectdetector for detecting a presence of an object in response to a loadingof an electromagnetic test signal generated by the object detector, theelectromagnetic test signal having a frequency corresponding to thefirst resonance frequency. The electromagnetic test signal may have afrequency equal to the first resonance frequency, or may typically bewithin 2 or 5% of the first resonance frequency. The electromagnetictest signal may in some embodiments be generated by a differentantenna/coil than the communication antenna. The electromagnetic testsignal may in some embodiments be generated by the communicationantenna, which may be a communication coil. The electromagnetic testsignal may in some embodiments be generated by a different antenna/coilthan the power transfer coil.

In many embodiments, the power transfer device may comprise an objectdetector for detecting a presence of an object comprising a resonancecircuit having a resonance frequency meeting a similarity criterion withrespect to the first resonance frequency.

In many embodiments, the power transfer device may comprise an objectdetector for detecting a presence of an object in response to a loadingof generated electromagnetic test signal having a frequency deviating byless than a threshold, say of 1%, from the first resonance frequency.

In some embodiments, the detection may be dependent on the resonance ofthe loading of the power transfer signal meeting a similarity criterionwith respect to the first resonance frequency. The similarity criterionmay be explicitly evaluated or may e.g. be implicit by a search for aresonance frequency being limited to an interval comprising the firstresonance frequency.

A similarity criterion may depend on the preferences and requirements ofthe individual embodiment. The similarity criterion may include arequirement that a detected resonance frequency of the object and thefirst resonance frequency differ by less than a threshold.

In accordance with an optional feature of the invention, the controlleris arranged to decouple the communication resonance circuit from thecommunicator and to couple it to the object detector during objectdetection time intervals.

This may allow particularly efficient performance and especiallyimproved detection performance. It may further reduce complexity ande.g. component requirements thereby reducing cost etc.

In accordance with an optional feature of the invention, the powertransfer device is the power receiver.

The invention may provide an improved power transmitter.

In accordance with an optional feature of the invention, the powertransfer device further comprises a synchronizer for synchronizing thecontroller to level variations of the power transfer signal.

This may provide efficient, yet low complexity, performance.

In accordance with an optional feature of the invention, there isprovided a wireless power transfer system comprising a power receiverand power transmitter as described above.

The invention may provide an improved wireless power transfer system.

According to an aspect of the invention there is provided a method ofoperation for a power transfer device for wireless power transfer from apower transmitter to a power receiver using an electromagnetic powertransfer signal, the power transfer device being one of the powertransmitter and the power receiver, and the power transfer signal duringa power transfer phase employing a repeating time frame comprising apower transfer time interval and an object detection time interval beingnon-overlapping with the power transfer time interval, a power limit ofthe power transfer signal being lower for the object detection timeinterval than for the power transfer time interval, the power transferdevice comprising a communication resonance circuit comprising acommunication antenna for transmitting or receiving the electromagneticcommunication signal, the method comprising: a power transfer circuitcomprising a power transfer coil receiving or generating the powertransfer signal during the power transfer time intervals; communicatingwith a complementary device being the other device of the power receiverand the power transmitter via an electromagnetic communication signal;controlling the communication resonance circuit being arranged toprovide a resonance at a first resonance frequency to the communicatorduring communication; and adapting the communication resonance circuitto not provide the resonance at the first resonance frequency to thecommunicator during object detection time intervals.

These and other aspects, features and advantages of the invention willbe apparent from and elucidated with reference to the embodiment(s)described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only,with reference to the drawings, in which

FIG. 1 illustrates an example of elements of an NFC reader and an NFCcard;

FIG. 2 illustrates an example of elements of a power transfer system inaccordance with some embodiments of the invention;

FIG. 3 illustrates an example of elements of a power transfer system inaccordance with some embodiments of the invention;

FIG. 4 illustrates an example of a repeating time frame for a powertransfer signal for a power transfer system in accordance with someembodiments of the invention;

FIG. 5 illustrates an example of elements of a power transfer system inaccordance with some embodiments of the invention;

FIG. 6 illustrates an example of elements of a power receiver for apower transfer system in accordance with some embodiments of theinvention; and

FIG. 7 illustrates an example of elements of a power receiver for apower transfer system in accordance with some embodiments of theinvention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following description focuses on embodiments of the inventionapplicable to a wireless power transfer system utilizing a powertransfer approach such as known from the Qi specification. However, itwill be appreciated that the invention is not limited to thisapplication but may be applied to many other wireless power transfersystems.

FIG. 2 illustrates an example of a power transfer system in accordancewith some embodiments of the invention. The power transfer systemcomprises a power transmitter 201 which includes (or is coupled to) atransmitter coil/inductor 203. The system further comprises a powerreceiver 205 which includes (or is coupled to) a receiver coil/inductor207.

The system provides an electromagnetic power transfer signal which mayinductively transfer power from the power transmitter 201 to the powerreceiver 205. Specifically, the power transmitter 201 generates anelectromagnetic signal, which is propagated as a magnetic flux by thetransmitter coil or inductor 203. The power transfer signal maytypically have a frequency between around 20 kHz to around 500 kHz, andoften for Qi compatible systems typically in the range from 95 kHz to205 kHz (or e.g. for high power kitchen applications, the frequency maye.g. typically be in the range between 20 kHz to 80 kHz). Thetransmitter coil 203 and the power receiving coil 207 are looselycoupled and thus the power receiving coil 207 picks up (at least partof) the power transfer signal from the power transmitter 201. Thus, thepower is transferred from the power transmitter 201 to the powerreceiver 205 via a wireless inductive coupling from the transmitter coil203 to the power receiving coil 207. The term power transfer signal ismainly used to refer to the inductive signal/magnetic field between thetransmitter coil 203 and the power receiving coil 207 (the magnetic fluxsignal), but it will be appreciated that by equivalence it may also beconsidered and used as a reference to an electrical signal provided tothe transmitter coil 203 or picked up by the power receiving coil 207.

In the example, the power receiver 205 is specifically a power receiverthat receives power via the receiver coil 207. However, in otherembodiments, the power receiver 205 may comprise a metallic element,such as a metallic heating element, in which case the power transfersignal directly induces eddy currents resulting in a direct heating ofthe element.

The system is arranged to transfer substantial power levels, andspecifically the power transmitter may support power levels in excess of500 mW, 1 W, 5 W, 50 W, 100 W or 500 W in many embodiments. For example,for Qi corresponding applications, the power transfers may typically bein the 1-5 W power range for low power applications (the basic powerprofile), up to 15 W for Qi specification version 1.2, in the range upto 100 W for higher power applications such as power tools, laptops,drones, robots etc., and in excess of 100 W and up to more than 2400 Wfor very high power applications, such as e.g. kitchen applications.

In the following, the operation of the power transmitter 201 and thepower receiver 205 will be described with specific reference to anembodiment generally in accordance with the Qi Specification (except forthe herein described (or consequential) modifications and enhancements)or suitable for the higher power kitchen specification being developedby the Wireless Power Consortium. In particular, the power transmitter201 and the power receiver 205 may follow, or substantially becompatible with, elements of the Qi Specification version 1.0, 1.1 or1.2 (except for the herein described (or consequential) modificationsand enhancements).

In wireless power transfer systems, the presence of an object (typicallya conductive element extracting power from the power transfer signal andnot being part of the power transmitter 201 or the power receiver 205,i.e. being an unintended, undesired, and/or interfering element to thepower transfer) may be highly disadvantageous during a power transfer.Such an undesired object is in the field known as a foreign object.

A foreign object may not only reduce efficiency by adding a power lossto the operation but may also degrade the power transfer operationitself (e.g. by interfering with the power transfer efficiency orextracting power not directly controlled e.g. by the power transferloop). In addition, the induction of currents in the foreign object(specifically eddy currents in the metal part of a foreign object) mayresult in an often highly undesirable heating of the foreign object.

In order to address such scenarios, wireless power transfer systems suchas Qi include functionality for foreign object detection. Specifically,the power transmitter comprises functionality seeking to detect whethera foreign object is present. If so, the power transmitter may e.g.terminate the power transfer or reduce the maximum amount of power thatcan be transferred.

Current approaches proposed by the Qi Specifications are based ondetecting a power loss (by comparing the transmitted and the reportedreceived power) or detecting degradations in the quality Q of the outputresonance circuit.

FIG. 3 illustrates the power transfer system of FIG. 2 in more detail.

In the example, the power transmitter 201 includes a driver 301 whichcan generate a drive signal that is fed to a power resonance circuit 303which includes the transmitter coil 203. The transmitter coil 203generates the electromagnetic power transfer signal, which can provide apower transfer to the power receiver 205. The power transfer signal isprovided during power transfer time intervals of the power transferphase.

The driver 301 may typically comprise an output circuit in the form ofan inverter, typically formed by driving a full or half bridge as willbe well known to the skilled person.

The power transmitter 201 further comprises a power transmittercontroller 305 which is arranged to control the operation of the powertransmitter 201 in accordance with the desired operating principles.Specifically, the power transmitter 201 may include many of thefunctionalities required to perform power control in accordance with theQi Specifications.

The power transmitter controller 305 is in particular arranged tocontrol the generation of the drive signal by the driver 301, and it canspecifically control the power level of the drive signal, andaccordingly the level of the generated power transfer signal. The powertransmitter controller 305 comprises a power loop controller controllinga power level of the power transfer signal in response to the powercontrol messages received from the power receiver 205 during the powercontrol phase.

The receiver coil 207 is part of a power receiver circuit 307 which maytypically comprise one or more capacitors forming a resonance circuitwith the receiver coil 207. During power transfer, a current is inducedin the receiver coil 207 and the power receiver circuit 307 in this wayextracts power from the power transfer signal. The signal is coupled toa power converter or power extractor 309 which is arranged to processand control the extracted power and feed this to a load 311. The powerextractor 309 may typically include rectifiers, voltage or currentcontrollers etc. which will be well known to the skilled person. Thepower extractor 309 may provide a power control path which converts thepower extracted by the receiver coil 207 into a suitable supply for aload 311, such as e.g. a battery being charged, or a device beingpowered.

The power receiver further comprises a power receiver controller 313which may include various power receiver controller functionalityrequired to perform power transfer, and in particular functions requiredto perform power transfer in accordance with the Qi specifications.

The power transmitter 201 and the power receiver 205 further includemeans for communicating with each other. In the example, thecommunication is not (or at least not exclusively) achieved bymodulating and demodulating the power transfer signal but rather aseparate short-range communication system is used. The short-rangecommunication system may typically have a range of no more than 1 meter,and often no more than 50 cm or even 20 cm. The short-rangecommunication system typically uses a carrier frequency substantiallyhigher than a frequency and typically at least 10 times higher than thecarrier frequency of the power transfer signal. In many embodiments, thecarrier frequency is at least 1 MHz, and often at least 10 MHz.

In the specific example, the power receiver 205 and the powertransmitter 201 communicate using an NFC communication approach. In someembodiments, only some of the communication will be via the NFCcommunication system whereas other communication may be by other means,e.g. using the power transfer signal as a communication carrier. Forexample, the NFC communication approach may be used to read powerreceiver specific parameters using the NFC communication while usinge.g. load modulation of the power transfer signal to communicate powercontrol error messages etc.

In the example, the power transmitter 201 comprises a first communicator315 which is arranged to communicate with the power receiver 205 via anelectromagnetic communication signal.

The first communicator 315 is coupled to a first communication resonancecircuit 317 which comprises a first communication antenna 319 thatgenerates the electromagnetic communication signal. The firstcommunicator 315 may specifically generate a communication drive signalwhich is fed to the first communication resonance circuit 317. The firstcommunication resonance circuit 317 is a load for the first communicator315 and it provides a resonance at a given first resonance frequency.Typically, the first communication resonance circuit 317 comprises atuned circuit which includes the first communication antenna 319. Forexample, the first communication antenna 319 may be a coil which iscoupled with one or more capacitors to form a resonance/tuned circuit.

The resonance circuit is typically tuned to the carrier frequency of theelectromagnetic communication signal resulting in the firstcommunication resonance circuit 317 providing a very efficient antennafor the first communicator 315. In practice, there may be somediscrepancy between the first resonance frequency and the carrierfrequency e.g. due to component tolerances and variations, the presenceof conductive material etc. However, typically it is possible to keepthe first resonance frequency within 5%, and often within 1%, of thecarrier frequency.

Correspondingly, the power receiver 205 comprises a second communicationresonance circuit 321 which is coupled to a second communicator 323. Thesecond communication resonance circuit 321 is complementary to the firstcommunication resonance circuit 317 and correspondingly comprises acommunication antenna (referred to as the second communication antenna325). The second communication antenna 325 is arranged to receive theelectromagnetic communication signal in that a current is induced in thesecond communication antenna 325.

The second communication resonance circuit 321 is coupled to the secondcommunicator 323 and provides an impedance with a resonance at a givensecond resonance frequency. The second communication resonance circuit321 accordingly provides a source with an impedance/load to the secondcommunicator 323 which has a resonance at a given second resonancefrequency. Typically, the second communication resonance circuit 321comprises a tuned circuit which includes the second communicationantenna 325. For example, the second communication antenna 325 may be acoil which is coupled with one or more capacitors to form aresonance/tuned circuit.

The resonance circuit is typically tuned to the carrier frequency of theelectromagnetic communication signal and to the first resonancefrequency resulting in the second communication resonance circuit 321providing a very efficient antenna. In practice, there may be somediscrepancies between the frequencies e.g. due to component tolerancesand variations, the presence of conductive material etc. However,typically it is possible to keep the second resonance frequency within5%, and often within 1%, of the carrier frequency and/or the firstresonance frequency.

In the example, the short-range communication uses load modulation tocommunicate from the power receiver 205 to the power transmitter 201.The first communicator 315 generates a drive signal for the firstcommunication resonance circuit 317 resulting in the electromagneticcommunication signal being generated. The second communicator 323 variesthe load of the second communication resonance circuit 321 dependent onthe data to be communicated to the power transmitter 201. This loadvariation results in variations in the drive signal (e.g. currentvariations) which can be detected by the first communicator 315.

The short-range communication may specifically use an NFC approach withthe first communicator 315/first communication resonance circuit 317implementing the functionality corresponding to an NFC reader and thesecond communicator 323/second communication resonance circuit 321implementing the functionality corresponding to an NFC card or tag.Thus, the power receiver 205 may effectively emulate an NFC card therebyallowing it to be read by the NFC reader function of the powertransmitter 201.

In many embodiments, such as those using the NFC approach, the carrierfrequency is (nominally) 13.56 MHz and thus the first and secondresonance frequencies are (close to) 13.56 MHz.

In order to detect the presence of unexpected metallic (conductive)elements, such as keys or coins, being heated by being exposed to alarge magnetic field, typical power transmitters tend to include foreignobject detection functionality specifically aimed at detecting whetherany undesired conductive elements are likely to be present within thegenerated electromagnetic field. Such foreign object detection isconventionally based on evaluating the quality factor of the powerresonance frequency or unaccounted for power loss for the power transfersignal. However, whereas this may provide acceptable performance in manyscenarios and for many types of objects, it has been found that there isa particular problem with smart cards, such as NFC or RFID cards, asthese tend to comprise only small amounts of metal which is difficult todetect reliably.

This tends to be a problem in practice as such cards are susceptible tobe damaged by a strong magnetic field. For example, an NFC card may bedamaged by a strong electromagnetic field with frequency in the rangebetween of 20-200 kHz despite being arranged to use a carrier frequencyof 13.56 MHz. For example, a strong magnetic field may result in arectified voltage Vcc which may be so high that it destroys the NFCchip. Such damage may occur even after very short exposure to themagnetic field, such as e.g. after only 10-20 msec.

To prevent that such smart cards are damaged by a strong magnetic fieldgenerated by the power transmitter, it has been proposed for powertransmitters to comprise a so-called Smart Card Detection (SCD) systemaimed directly at detecting such cards. More generally, some powertransmitters include a resonance device detection circuit that isarranged to detect the presence of a resonance circuit at a givenfrequency (or close to that, i.e. within a suitable frequency range).Such a resonance detection function may specifically perform a detectionwhich is based on specific parameters of the device being detected, suchas specifically be aimed at detecting the presence of a resonancefrequency having a given resonance frequency. The resonance devicedetection circuit may specifically be arranged to generate a test signalthat will be particularly affected by the presence of a resonancecircuit with the expected parameters, and specifically with the expectedresonance frequency.

As a specific example, a test signal may be generated with a frequencycorresponding to the nominal resonance frequency. For example, a testdrive signal with a frequency corresponding to the nominal frequency maybe fed to a resonance circuit tuned to the nominal resonance frequencyand with a coil arranged to operate as an antenna. If a device with aresonance circuit tuned to (or close to) the nominal resonance frequencyis brought into the proximity of the detection circuit, it will have astrong impact on the detection resonance circuit and this can bedetected, e.g. as changes in the current of the drive signal.

Such an approach may allow a detection of the presence of a devicecomprising such a resonance circuit even in situations whereconventional power loss or Q factor foreign object detection approachesare not useful due to requiring a relatively high amount of metal to bepresent for detection. For example, it allows detection of e.g. smartcards such as NFC cards.

This has a large practical impact as smart cards may be damaged by beingexposed to a strong AC magnetic field, even at a significantly differentfrequency. This is in particular an issue for higher power levelwireless power transfer applications, such as those being developed forkitchen applications where e.g. a kettle or pan may be powered. In thosesystems, it may be necessary for a detection system for smart cardswhich is highly sensitive.

The power transmitter 201 of FIG. 3 comprises an object detector in theform of a resonance device detector 327 which is arranged to detect thepresence of a resonance circuit (having a nominal/predeterminedresonance frequency (e.g. which can be assumed to be within a givenfrequency range comprising a nominal resonance frequency that theresonance device detector 327 is aimed at detecting), and thus a devicecomprising such a resonance circuit.

In the example, the resonance device detector 327 comprises a detectionresonance circuit 329 which is tuned to the nominal detection resonancefrequency. In the example, the resonance device detector 327 is a smartcard detector arranged to detect e.g. NFC cards and accordingly it istuned to (around) 13.56 MHz. The detection resonance circuit 329 iscoupled to a driver/oscillator 331 which is arranged to generate acarrier signal which has a frequency of the nominal detection resonancefrequency. Thus, the oscillator in the specific example generates a13.56 MHz drive signal and feeds it to the detection resonance circuit329.

The resonance device detector 327 further comprises a detectionprocessor 332 which is coupled to the oscillator 331. The detectionprocessor 332 is arranged to evaluate a parameter of the drive signal,such as for example the drive current. If the drive parameter deviatesfrom that expected in the case of no smart card being present, thedetection processor 332 proceeds to determine that a smart card ispotentially present. If so, this detection result is fed to the powertransmitter controller 301 which proceeds to take appropriate action,e.g. it may terminate a power transfer or reduce the maximum powerlevel.

The detection processor 332 may specifically be arranged to detect thepresence of an object that has a resonance frequency corresponding tothat of the detection resonance circuit 329 (and that of the firstcommunication resonance circuit 317), i.e. corresponding to the firstresonance frequency. The object detector may detect a presence of anobject comprising a resonance circuit having a resonance frequencycorresponding to the resonance frequency of the first communicationresonance circuit 317 by determining a coil current for a resonancecircuit (the detection resonance circuit 329) that has the sameresonance frequency and/or using a drive signal having the sameresonance frequency. The object detector/resonance device detector 327may be arranged to detect a presence of an object comprising a resonancecircuit having a resonance frequency corresponding to the firstresonance frequency by determining a loading of a generatedelectromagnetic test signal having a frequency deviating from the firstresonance frequency by less than a threshold (the threshold maytypically be e.g. 0.1%, 0.5%, 1%, 5%, or 10% of the first resonancefrequency). Typically the generated electromagnetic signal has afrequency substantially equal to the first resonance frequency. In theexample of FIG. 3 , the oscillator 331 is arranged to generate a drivesignal for the detection resonance circuit 329 such that theelectromagnetic (object detection) test signal is generated. The drivesignal is specifically generated to have a frequency substantially equalto the first resonance frequency.

In order to provide improved detection, the power transfer signal duringthe power transfer phase employs a repeating time frame which comprisesat least one power transfer time interval and at least one objectdetection time interval with these time intervals being non-overlapping.During the power transfer time intervals, the power transmitter 201transfers power to the power receiver 205 by generating a power transfersignal that has the required power level necessary to provide therequired power to the power receiver 205. Specifically, the power levelduring the power transfer time intervals is set in response to the powercontrol messages received from the power receiver 205.

The repeating time frame is typically a periodic repeating time frame.In many embodiments, each repeating time frame may have a duration of nomore than 0.5 second, 1 second, 2 seconds, or 5 seconds. The repeatingtime frame may specifically be a periodically repeating time frame witha period of no more than 0.5 second, 1 second, 2 seconds, or 5 seconds.

During the object detection time intervals, the power level is typicallyreduced substantially respectively to during the power transfer timeintervals. Specifically, the maximum power limit during the objectdetection time intervals is lower than during the power transfer timeintervals and typically to a much lower level. For example, during theobject detection time intervals, the maximum power level may berestricted to a limit which is less than 0.5 W, 1 W, 5 W, or 10 W. Thepower limit during the power transfer signal may typically be at least5, 10, 50, or a 100 times higher. Thus, the electromagnetic field of thepower transfer signal is substantially lower during the power transfertime intervals than during the object detection time intervals.

In many embodiments, the power level may be set to a fixed power levelduring the object detection time intervals, and specifically it may insome embodiments be set to substantially zero, i.e. the power transfersignal may be switched off. An advantage of such an approach is that theelectromagnetic field of the power transfer signal during the objectdetection time intervals is not only very low but also constant andpredictable.

An example of a repeating time frame is illustrated in FIG. 4 wherepower transfer time intervals are indicated by PT and object detectiontime intervals are indicated by D. In the example, each time frame FRMcomprises only one object detection time interval and one power transfertime interval and these (as well as the time frame itself) have the sameduration in each frame. However, it will be appreciated that in otherembodiments, other time intervals may also be included in a time frame(such as e.g. communication intervals) or a plurality of objectdetection time intervals and/or power transfer time intervals may beincluded in each time frame. Furthermore, the duration of the differenttime intervals (and indeed the time frame itself) may in someembodiments vary dynamically.

The repeating time frame may in many embodiments be an invariant, fixed,constant, or even predetermined time frame. In many embodiments, theduration of each time frame may be constant (at least for someconsecutive time frames) and the timing of the object detection timeintervals within the time frames may be invariant, fixed, constant, oreven predetermined. In many embodiments, such as in the example of FIG.4 , the individual time frames are identical. The timing of therepeating time frame, and typically of the object detection timeintervals and/or the power transfer time intervals is fixed andconstant.

In the system, the power transmitter is thus arranged to perform powertransfer during the power transfer time interval of the time frames ofthe power transfer phase. Specifically, during these time intervals, thepower transmitter 201 and the power receiver 205 may operate a powercontrol loop (the power control loop may be based on communicationwithin the power transfer signal time interval or may e.g. be based oncommunication outside of the power transfer signal time interval, suchas in dedicated communication time intervals. For example, each foreignobject time interval may be separated by a plurality of alternatingpower transfer signal time intervals and communication time intervals).Thus, the level of the power being transferred may be dynamicallyvaried.

In some embodiments, the power receiver 205 may also be arranged toreduce the load of the power transfer signal during the object detectiontime intervals. For example, it may disconnect or decouple a load toreduce the power extracted from the power transfer signal during theobject detection time intervals.

In the approach, the object detection by the resonance device detector327 and the power transfer is thus separated in the time domain therebyresulting in reduced cross-interference from the power transfer to theobject/card detection. Thus, the interference caused by the powertransfer signal to the resonance card/smart card detection is reduced.Further, the variability and uncertainty resulting from variations inthe operating conditions for the power transfer can be isolated from theobject detection resulting in a more reliable and accurate detectionperformance.

However, the Inventors have realized that despite this approach,detection of resonance devices tend to not be as accurate as desired inmany scenarios. They have further realized that the detectionperformance can be improved by controlling the operation of thecommunication circuitry, and specifically by controlling the firstcommunication resonance circuit 317 and/or the second communicationresonance circuit 321. In the system of FIG. 1 , the power transmitter201 and/or the power receiver 205 comprises means for adapting theircommunication resonance circuit such that it provides a resonance at afirst resonance frequency during communication but does not provide thisresonance at the first resonance frequency during the object detectiontime intervals.

Specifically, the power transmitter 201 comprises a first controller 333which is arranged to control the first communication resonance circuit317 such that the first communication resonance circuit 317 provides thefirst resonance frequency during times when the short-rangecommunication system is used for communication with the power receiver205 but not during the object detection time intervals.

The communication using the short-range communication system isperformed outside of the object detection time intervals. Thus, thefirst communicator 315 and the second communicator 323 are arranged toperform the communication outside of the object detection timeintervals. The timing of the repeating time frame may be controlled bythe power transmitter controller 305 and a timing signal may be fed tothe first communicator 315 to control when the communication occurs. Insome embodiments, the repeating time frame may include dedicatedcommunication time intervals being non-overlapping with the powertransfer time intervals and the object detection time intervals, andspecifically the power level of the power transfer signal may be reducedduring such communication time intervals (corresponding to the approachfor the object detection time intervals). In other embodiments,communication by the first communicator using the first communicationresonance circuit 317 may be performed simultaneously with the powertransfer, i.e. during the power transfer time intervals.

During communication by the first communicator 315 (specifically duringcommunication time intervals whether overlapping or non-overlapping withthe power transfer time intervals), the first controller 333 controlsthe first communication resonance circuit 317 to provide the resonanceat the first resonance frequency and thus allows for optimizedcommunication. However, during the object detection time intervals, thefirst controller 333 controls the first communication resonance circuit317 to not provide this resonance frequency. This may be achieved indifferent ways.

In some embodiments, the first communication resonance circuit 317 maybe detuned to change the resonance frequency during the object detectiontime intervals. Thus, the first controller 333 may control the firstcommunication resonance circuit 317 to change the first communicationresonance circuit 317 such that it has a different resonance frequency,referred to as the modified resonance frequency, during the objectdetection time intervals than during the communication time intervals.

The resonance frequency may for example be changed by switching in (orout) an additional resonance component, such as for example a capacitorforming part of the resonance circuit. For example, during the objectdetection time intervals, the first controller 333 may control the firstcommunication resonance circuit 317 to switch in an additional capacitorwhich changes the effective resonance capacitance of the resonancecircuit thereby changing the resonance frequency.

The resonance frequency will typically be changed relativelysubstantially, and in most embodiments will be changed such that themodified resonance frequency is substantially different than the first.In most embodiments, the modified resonance frequency is outside a rangefrom 90% of the first resonance frequency to 110% of the first resonancefrequency. This will tend to result in a significantly reduced impact ofthe first communication resonance circuit 317 on the object detectionsby the resonance device detector 327. In some embodiments, the deviationmay be no less than 20%, 50% or even 100%.

In some embodiments, the first controller 333 may be arranged to controlthe first communication resonance circuit 317 to not provide a resonanceto the first communicator 315 during the object detection timeintervals. This may be achieved e.g. by modifying the firstcommunication resonance circuit 317 to not have any resonance or bydecoupling/disconnecting the first communication resonance circuit 317from the first communicator 315.

In the former case, the first communication resonance circuit 317 mayfor example comprise a switch which disconnects the first communicationantenna from the rest of the resonance circuit it is part of, or e.g. bydisconnecting the resonance capacitor(s) from the resonance circuit.This will effectively change the circuit to not form a resonance circuitduring the object detection time intervals. The switch may then becontrolled by the first controller 333.

In such embodiments, the first communication resonance circuit 317 maythus still oscillate at the first resonance circuit but it will bedecoupled from the first communicator 205. In many such embodiments, thepower transmitter 201 may further be arranged to couple the powerreceiver circuit 307 to the resonance device detector 327 during theobject detection time intervals.

For example, as illustrated in FIG. 5 , the power transmitter 201 maycomprise an additional switch 501 which switches the first communicationresonance circuit 317 from being coupled to the first communicator 205to being coupled to the resonance device detector 327, and specificallyto the oscillator 331. The switch is controlled by the first controller333 and is arranged to switch such that it couples the firstcommunication resonance circuit 317 to the first communicator 315 duringcommunication (e.g. during communication time intervals of the repeatingtime frame) and such that it couples the first communication resonancecircuit 317 to the resonance device detector 327 during the objectdetection time intervals.

Such an approach may provide particularly efficient operation as itallows for circuitry to be reused between very different functionsthereby allowing reduced cost, complexity, size etc.

The Inventors have realized that the approaches of modifying or removingthe resonance frequency of the communication functionality whenperforming object detection for a resonance device or card may provideimproved resonance device detection as it removes or reduces theinterference between the communication function and the detectionfunctions. This is particularly for many practical embodiments whereinthe communication uses a frequency corresponding to the resonancefrequency the resonance device detector 327 is seeking to detect. Forexample, it is particularly advantageous when trying to detect NFC cardsin wireless power transfer systems that use an NFC approach forcommunication between the power receiver and the power transmitter.

In many embodiments, the tuned communication antenna can be made“invisible” to the object detection circuit, and especially in someembodiments this can be achieved by combining the object detection andcommunication antennas/coils into an integrated system.

The detection and communication performances are improved by theapplication of a time division approach where these functions areperformed at different times. However, the Inventors have realized thatthis in itself often does not achieve optimal detection performance inpractice. They have realized that the removal of the resonance frequencyfor the communication circuitry during resonance object/device detectioncan substantially improve detection performance.

Practical experiments for Qi type systems have shown that using timedivision between communication and detection may provide improvedperformance but that it is still not as reliable as desired. It mayspecifically require quite long detection intervals in order to achievesufficiently reliable detection performance. In many such systems, thedetection time may in practice be in the order of 30-40 msec whereas thedescribed approach may reduce this to around 0.2-2.0 msec. This isparticularly advantageous and significant in many embodiments as itbrings the detection time in line with that typically required fornon-resonance object detections, such as specifically conventionalforeign object detection based on power loss or Q-factor measurements.This in particular allows these operations to be performedsimultaneously and specifically means the object detection timeintervals may be used simultaneously both for resonance device detectionand for foreign object detection based on detection of conductivematerial/metal.

In some embodiments, the power transmitter 201 may also comprise anon-resonance foreign object detector arranged to perform foreign objectdetection during the object detection time intervals. The non-resonanceforeign object detector may be arranged to perform power loss and/orquality factor foreign object detection. The foreign object detectionmay be a detection of the presence of a conductive element, such as thepresence of metal.

In many embodiments the power receiver 205 may alternatively oradditionally also be arranged to modify the resonance operation of thesecond communication resonance circuit 321 during the object detectiontime intervals relative to when communication is being performed.

The power receiver 205 comprises a second controller 335 which isarranged to control the second communication resonance circuit 321. Thesecond communication resonance circuit 321 may operate correspondinglyto the first communicator 205 and may specifically control the secondcommunication resonance circuit 321 to change its resonance frequency orto completely prevent resonance during the object detection timeintervals.

The comments and descriptions previously provided with respect to thefirst communication resonance circuit 317 and the first controller 333apply mutatis mutandis to the second communication resonance circuit 321and the second controller 335.

Thus, the second controller 335 may be arranged to control the secondcommunication resonance circuit 321 to switch in or out a resonancecomponent such as a series or parallel second capacitor thereby changingthe resonance frequency. In other embodiments, the second controller 335may control the second communication resonance circuit 321 to disconnectthe second communication antenna 325 from the corresponding resonancecapacitor thereby preventing resonance.

Whereas the power transmitter 201 generates the power transfer signaland thus inherently has knowledge of the timing of the repeating timeframe employed, the power receiver 205 may not have this informationavailable. Therefore, in many embodiments, the power receiver 205 maycomprise functionality for synchronizing the operation of the secondcontroller 335 to the repeating time frame of the power transfer signal.

The power receiver controller 313 may for example synchronize the secondcontroller 335 to level variations (variations in the level) of thepower transfer signal. The power level of the induced signal is duringthe power transfer time intervals typically much higher than during theobject detection time intervals. Therefore, a power level transition canbe detected between the time intervals and this can be used tosynchronize a local time base to the power transfer signal, and thus canbe used to synchronize the switching of the second communicationresonance circuit 321 to the repeating time frame of the power transfersignal.

In some embodiments, during the object detection time intervals, thepower transmitter 201 has reduced power of the power transfer signal toa level where the power receiver 205 receives substantially no power.The power transmitted by the power transmitter 201 will in this casemainly be absorbed by a foreign object in proximity of the powertransmitter 201. This can be measured with a much higher accuracybecause the uncertainty of the power transferred towards the powerreceiver 205 is not in the equation anymore. Therefore, improvednon-resonance based foreign object detection can be achieved. Inaddition, improved resonance based object detection (by the resonancedevice detector 327) can be achieved as the interference caused by thepower transfer signal is also reduced at the substantially differentfrequency of the resonance (e.g. 13.56 MHz). Further, by removing theresonance of the second communication resonance circuit 321 (and thefirst communication resonance circuit 317), the interference of thecommunication circuitry is reduced.

As an example, if the load of the power receiver 205 is a battery with acertain battery voltage Ubatt as exemplified in FIG. 6 , power transferto the power receiver 205 can effectively be disconnected by reducingthe power signal/magnetic field generated by the power transmitter 201.This is the case when the induced voltage U_Rx at the input of thereceiver's rectifier is lower than the battery voltage Ubatt. In thatcase the diodes D1 to D4 of the rectifier bridge do not conduct and nocurrent flows to the battery (the load). The diodes act like a passivedisconnection switch. Although the power transmitter 201 has reduced thetransferred power during the object detection time interval, the drivesignal for switching the second communication resonance circuit 321(i.e. the timing signal representing the repeating time frame) caneasily be derived from the induced voltage U_Rx by means of amplitudedemodulation.

As another example which is illustrated in FIG. 7 , if the load of thepower receiver 205 is not a battery but an e.g. a resistive load, thedisconnection of the load will not take place automatically by the powerlevel being reduced. The diodes D1 to D4 of the rectifier bridge willstay in conduction mode. With switch S3 the diodes D1 to D4 can bebrought in non-conduction mode in order to effectively disconnect theload Rload. The drive signal for switching the second communicationresonance circuit 321 can again be derived from the induced voltage U_Rxby means of amplitude demodulation.

In many embodiments, the exact timing of the switching of the operationof the first communication resonance circuit 317 and the secondcommunication resonance circuit 321 is not critical. In manyembodiments, the first controller 333 and the second controller 335 maybe arranged to switch the first communication resonance circuit 317 andsecond communication resonance circuit 321 to not provide resonance atthe first frequency with a suitable time margin before the start of anobject detection time interval and with a suitable time margin after theend of an object detection time interval. Thus, no resonance at thefirst resonance frequency is present during the object detection timeintervals.

Similarly, the first controller 333 and the second controller 335 mayswitch the first communication resonance circuit 317/secondcommunication resonance circuit 321 to the first resonance frequencyprior to starting any communication and may not switch away until aftercommunication.

In many embodiments, a repeating time frame may comprise both an objectdetection time interval and a communication time interval with powertransfer intervals in-between. In such an embodiment, the firstcontroller 333/second controller 335 may in principle perform theswitching at any time during the power transfer intervals. As these aretypically very long compared to both the communication time intervalsand the object detection time intervals, it tends to not provide stricttiming requirements and allow even relatively low accuracysynchronization to be sufficient. However, in many embodiments, it isdesirable to maximize the time in which the resonance circuits have aresonance at the first resonance frequency and therefore switching mayin many embodiments be performed shortly before (or even upon entering)the object detection time interval and shortly after (or even uponexiting) the object detection time interval. This may allow the firstcommunicator 315 to provide a communication carrier during most of therepeating time frame which may be useful in embodiments wherein somefunctionality of the power receiver (e.g. the second communicator) arepowered by energy extracted from the communication signal.

Although the previous description has focused on the detection of smartcards, it will be appreciated that it may be used for detection of otherobjects, and specifically of other resonance devices having a resonancewith respect to a magnetic field in which the device is present.

It will also be appreciated that in many embodiments both the powertransmitter and the power receiver will switch the resonance frequencyof the communication resonance circuits but that in some embodimentsonly one of the power transmitter and the power receiver may apply thisapproach. This may still provide improved detection and reduce theinterference between the communication function and the detectionfunction.

It will be appreciated that the above description for clarity hasdescribed embodiments of the invention with reference to differentfunctional circuits, units and processors. However, it will be apparentthat any suitable distribution of functionality between differentfunctional circuits, units or processors may be used without detractingfrom the invention. For example, functionality illustrated to beperformed by separate processors or controllers may be performed by thesame processor or controllers. Hence, references to specific functionalunits or circuits are only to be seen as references to suitable meansfor providing the described functionality rather than indicative of astrict logical or physical structure or organization.

The invention can be implemented in any suitable form includinghardware, software, firmware or any combination of these. The inventionmay optionally be implemented at least partly as computer softwarerunning on one or more data processors and/or digital signal processors.The elements and components of an embodiment of the invention may bephysically, functionally and logically implemented in any suitable way.Indeed, the functionality may be implemented in a single unit, in aplurality of units or as part of other functional units. As such, theinvention may be implemented in a single unit or may be physically andfunctionally distributed between different units, circuits andprocessors.

Although the present invention has been described in connection withsome embodiments, it is not intended to be limited to the specific formset forth herein. Rather, the scope of the present invention is limitedonly by the accompanying claims. Additionally, although a feature mayappear to be described in connection with particular embodiments, oneskilled in the art would recognize that various features of thedescribed embodiments may be combined in accordance with the invention.In the claims, the term comprising does not exclude the presence ofother elements or steps.

Furthermore, although individually listed, a plurality of means,elements, circuits or method steps may be implemented by e.g. a singlecircuit, unit or processor. Additionally, although individual featuresmay be included in different claims, these may possibly beadvantageously combined, and the inclusion in different claims does notimply that a combination of features is not feasible and/oradvantageous. Also the inclusion of a feature in one category of claimsdoes not imply a limitation to this category but rather indicates thatthe feature is equally applicable to other claim categories asappropriate. Furthermore, the order of features in the claims do notimply any specific order in which the features must be worked and inparticular the order of individual steps in a method claim does notimply that the steps must be performed in this order. Rather, the stepsmay be performed in any suitable order. In addition, singular referencesdo not exclude a plurality. Thus, references to “a”, “an”, “first”,“second” etc. do not preclude a plurality. Reference signs in the claimsare provided merely as a clarifying example shall not be construed aslimiting the scope of the claims in any way.

The invention claimed is:
 1. A power transmitter device comprising: a power transfer circuit, wherein the power transfer circuit comprises a power transfer coil, wherein the power transfer coil is arranged to generate a power transfer signal during at least one power transfer interval(s), wherein the at least one power transfer interval(s) is a portion of at least one repeating frame(s), wherein the at least one repeating frame(s) comprises the at least one power transfer interval(s) and at least one object detection interval(s); a communicator circuit, wherein the communicator circuit is arranged to communicate with a power receiver via a communication signal; a communication resonance circuit, wherein the communication resonance circuit comprising a communication antenna, wherein the communication antenna is arranged to transmit and receive the communication signal, wherein the communication resonance circuit is arranged to provide a resonance at a first resonance frequency for the communicator circuit; and a controller circuit, wherein the controller circuit is arranged to adapt the communication resonance circuit to not provide the resonance for the communicator circuit during the at least one object detection interval(s).
 2. The power transmitter device of claim 1, wherein the controller circuit is arranged to decouple the communication resonance circuit from the communicator circuit during the at least one object detection interval(s).
 3. The power transmitter device of claim 1, wherein the controller circuit is arranged to detune the communication resonance circuit from the first resonance frequency during the at least one object detection interval(s).
 4. The power transmitter device of claim 3, wherein the controller circuit is arranged to detune the communication resonance circuit to a second resonance frequency during the at least one object detection interval(s).
 5. The power transmitter device of claim 4, wherein the second frequency is outside a frequency range, wherein the frequency range is from 90% to 110% of the first resonance frequency.
 6. The power transfer device of claim 3, wherein the controller circuit is arranged to change a resonance capacitance of the communication resonance circuit during the at least one object detection interval(s) to during communication.
 7. The power transmitter device of claim 1, wherein the first resonance frequency deviates by no more than 5% from a carrier frequency of the communication signal.
 8. The power transmitter device of claim 7, further comprising an object detector, wherein the object detector comprises comprising a resonance circuit, wherein the resonance circuit is arranged to have a resonance frequency corresponding to the first resonance frequency, wherein the object detector is arranged to detect a presence of an object.
 9. The power transmitter device of claim 7, wherein the controller circuit is arranged to decouple the communication resonance circuit from the communicator circuit during the at least one object detection time interval(s), wherein the controller circuit is arranged to couple the communicator circuit to the object detector during the at least one object detection interval(s).
 10. The power transmitter device of claim 1, further comprising a synchronizer, wherein the synchronizer is arranged to synchronize the communication resonance circuit by the controller circuit to level variations of the power transfer signal.
 11. A method of operation of a power transmitter device for wireless power transfer from a power transmitter to a power receiver using an power transfer signal, wherein the power transfer signal during a power transfer phase uses at least one repeating frame(s), wherein the at least one repeating time frame(s) comprise at least one power transfer interval(s) and at least one object detection interval(s), wherein a power limit of the power transfer signal is lower for the at least one object detection interval(s) than for the power transfer time interval, wherein the power transmitter device comprises a communication resonance circuit wherein the communication resonance circuit comprises a communication antenna and a power transfer circuit, wherein the communication antenna is arranged to transmit and receive a communication signal, wherein the power transfer circuit comprises a power transfer coil, the method comprising: generating the power transfer signal during the at least one power transfer interval(s) using the power transfer coil; communicating with a power receiver via an communication signal; controlling the communication resonance circuit to provide a resonance at a first resonance frequency to the communicator circuit; and adapting the communication resonance circuit to not provide the resonance at the first resonance frequency to the communicator circuit during the at least one object detection interval(s).
 12. A computer program stored on a non-transitory medium, wherein the computer program when executed on a processor performs the method as claimed in claim
 11. 13. A power receiver device comprising: a power transfer circuit, wherein the power transfer circuit comprises a power transfer coil, wherein the power transfer coil is arranged to receive a power transfer signal during at least one power transfer interval(s), wherein the at least one power transfer interval(s) is a portion of at least one repeating frame(s), wherein the at least one repeating frame(s) comprises the at least one power transfer interval(s) and at least one object detection interval(s); a communicator circuit, wherein the communicator circuit is arranged to communicate with a power transmitter via a communication signal; a communication resonance circuit, wherein the communication resonance circuit comprising a communication antenna, wherein the communication antenna is arranged to transmit and receive the communication signal, wherein the communication resonance circuit is arranged to provide a resonance at a first resonance frequency for the communicator circuit; and a controller circuit, wherein the controller circuit is arranged to adapt the communication resonance circuit to not provide the resonance for the communicator circuit during the at least one object detection interval(s).
 14. The power receiver device of claim 13, wherein the controller circuit is arranged to decouple the communication resonance circuit from the communicator circuit during the at least one object detection interval(s).
 15. The power receiver device claim 13, wherein the controller circuit is arranged to detune the communication resonance circuit from the first resonance frequency during the at least one object detection interval(s).
 16. The power receiver device of claim 15, wherein the controller circuit is arranged to detune the communication resonance circuit to a second resonance frequency during the at least one object detection interval(s).
 17. The power receiver device of claim 16, wherein the second frequency is outside a frequency range, wherein the frequency range is from 90% to 110% of the first resonance frequency.
 18. The power receiver device of claim 15, wherein the controller circuit is arranged to change a resonance capacitance of the communication resonance circuit during the at least one object detection interval(s) to during communication.
 19. The power receiver device of claim 13, wherein the first resonance frequency deviates by no more than 5% from a carrier frequency of the communication signal.
 20. The power receiver device of claim 13, further comprising a synchronizer, wherein the synchronizer is arranged to synchronize the communication resonance circuit by the controller circuit to level variations of the power transfer signal.
 21. A method of operation of a power receiver device for wireless power transfer from a power transmitter to a power receiver using an power transfer signal, wherein the power transfer signal during a power transfer phase uses at least one repeating frame(s), wherein the at least one repeating time frame(s) comprise at least one power transfer interval(s) and at least one object detection interval(s), wherein a power limit of the power transfer signal is lower for the at least one object detection interval(s) than for the power transfer time interval, wherein the power receiver device comprises a communication resonance circuit, wherein the communication resonance circuit comprises a communication antenna and a power transfer circuit, wherein the communication antenna is arranged to transmit and receive a communication signal, wherein the power transfer circuit comprises a power transfer coil, the method comprising: receiving the power transfer signal during the at least one power transfer interval(s) using the power transfer coil; communicating with a power receiver via an communication signal; controlling the communication resonance circuit to provide a resonance at a first resonance frequency to the communicator circuit; and adapting the communication resonance circuit to not provide the resonance at the first resonance frequency to the communicator circuit during the at least one object detection interval(s).
 22. A computer program stored on a non-transitory medium, wherein the computer program when executed on a processor performs the method as claimed in claim
 19. 